In general, pressure-sensitive adhesive (or PSA; the same applies hereinafter) has characteristics to be in a soft solid (viscoelastic) state in a room temperature range and adhere to adherend under some pressure. With the benefit of such properties, PSA has been preferably used in PSA sheet forms, for fixing components in portable electronics such as mobile phones, smartphones, and tablet PCs. It is also preferably used as PSA sheets that are applied, but removed afterwards. References disclosing this type of conventional art include Patent Documents 1 and 2. Both Patent Documents 1 and 2 disclose conventional art having extendable substrates. Patent Document 2 relates to a medical PSA sheet applied to the skin.